If you've ever played a windowed game, and the pointer isn't locked within the
screen, you may occasionally click outside the window by mistake. This is
super annoying if it happens in a tight spot, so here's a drop-in fix for the
problem.

Returns a readable stream, which emits the current position of the mouse
(relative to the element you've attached it to). The user can click on the
element to enable pointer lock - when not trapped, the stream will fall back to
the data it can get from the element's mousemove event.